1. Field of the Invention
The present invention relates to a process for removing heat-stable salts from ethanolamine gas purification process units. Ethanolamine units remove H.sub.2 S and CO.sub.2 from gaseous process streams. One objective of the invention is to improve the gas purification performance of the ethanolamine unit. Another objective is to control the corrosion rate of the unit. Both objectives are accomplished by the removal of heat-stable salts accumulating in the ethanolamine gas purification process units. For the purpose of this application, it is understood that the term "ethanolamine" is a generic term including, but not limited to, monoethanolamine, diethanolamine, triethanolamine and methyl diethanolamine.
2. Description of the Prior Art
The removal of hydrogen sulfide from gaseous streams, such as the waste gases liberated in the course of various chemical and industrial processes, for example, in the pulping of wood, natural gas and crude oil production and in petroleum refining, has become increasingly important in combating atmospheric pollution. Hydrogen sulfide-containing gases not only have an offensive odor, but such gases may cause damage to vegetation, painted surfaces and wildlife, as well as constitute a significant health hazard to humans. Government-wide regulations have increasingly imposed continuously lower tolerances on the content of hydrogen sulfide which can be vented to the atmosphere, and it is now imperative in many localities to remove virtually all the hydrogen sulfide under the penalty of an absolute ban on continuing operation of a plant or the like which produces the hydrogen sulfide-containing gaseous stream. Solutions of water and one or more of the ethanolamines are widely used in industry to remove hydrogen sulfide and carbon dioxide from gaseous streams.
Corrosion in ethanolamine units significantly increases both operating and maintenance costs. The mechanisms of corrosive attack include corrosion, corrosion-erosion and stress-corrosion cracking. Corrosion control techniques include the use of more expensive corrosion and erosion resistant materials, continuous periodic removal of corrosion-promoting agents in suspended solids from the solution by filtration or activated carbon adsorption and addition of corrosion inhibitors. (See Kohl, A. L. and Reisenfeld, F. C., Gas Purification, Gulf Publishing Co., Houston, 1979, pg. 91-105).
While corrosion resistant materials of construction and chemical corrosion inhibitors may reduce corrosive attack, neither addresses the progressive degradation in process unit performance caused by the accumulation of heat stable salts in the ethanolamine solution.
It is known that these heat stable salts may be separated from the enthanolamine solution by distillation. However, such separation is limited to relatively mild conditions of temperature and pressure to avoid thermal degradation of the ethanolamine solution. For a survey of the chemistry, engineering, and operational aspects of ethanolamine gas purification. (See K. F. Butwell, D. J. Kubek, and P. W. Sigmund, "Alkanolamine Treating", HYDROCARBON PROCESSING, Mar., 1982.)